: : Thermal expansion. If the water got warmer after you closed the valve, it would expand, with a corresponding increase in pressure (as long as you have no leaks and did not open any faucets). In a closed system, it takes very little expansion to raise the pressure significantly.

: : : This one has really got me puzzled. I shut off the water to the house I have The water pressure regulator comes first then the shut off valve then my pressure gauge comes next.The pressure guage went from 55 lbs to 100lbs this with the water shut off?????? What can cause the pressure to go up? Tom

: I noticed this would happen when someone would take a shower. I know my water heater has a pressure relief valve on it. How can the water be getting heated to cause thermal expansion? Is my water heater bad? Tom

If you isolate a water system (i.e. close your main valve, confining the water to a finite volume) and then the water is heated (or just warms up because your house is warmer than the water in the pipes), the pressure in the system will increase. So if you close the valve just after someone takes a shower, you have just introduced a lot of cold water into your water heater. The heater then warms this and the water expands, increasing the pressure. Nothing wrong with your heater. If the pressure rises above the setpoint of the relief valve, the valve should open and release enough water to lower the pressure in the system.

On a truly closed system (such as a hot water space heating system), this thermal expansion of water must be compensated for with an expansion tank, which has an air space (or a pressurized bladder) to absorb the additional water volume. In some cases, a domestic hot water system should have an expansion tank, particularly when codes require backflow prevention (a check valve) to prevent water from flowing backwards into the municipal water supply.